Ion exchange membranes transport cations or anions under an electrical or chemical potential. Ion exchange membranes have either negatively or positively charged groups attached to the polymeric material making up the bulk of the membrane. The counterion of each is the transferable ion. A cation exchange membrane will have fixed negative charges and mobile positively charged cations. Similarly, anion exchange membranes will have fixed positively charged groups and mobile negatively charged anions. Ion exchange membrane properties are controlled by the amount, type and distribution of the fixed ionic groups. These membranes may be described as strong acid and strong base, or weak acid and weak base membranes. Strong acid cation exchange membranes usually have sulfonic acid groups as the charged group, whereas for weak acid membranes, carboxylic acid typically makes up the fixed charged group. Quaternary and tertiary amines respectively produce the fixed positive charged groups in strong and weak base anion exchange membranes.
Among the most important applications of ion exchange membranes are desalination of water by electrodialysis (ED), as a power generating sources in reverse electrodialysis and as separators in fuels cells. Other applications include recovery of metal ions in the electroplating and metal finishing industries and various applications in the food and beverage industry.
Electrodialysis desalinates water by transferring ions and some charged organics through paired anion- and cation selective membranes under the motive force of a direct current voltage. An ED apparatus consists of electrically conductive and substantially water impermeable anion selective and cation selective membranes arranged as opposing walls of a cell. Adjacent cells form a cell pair. Membrane stacks consist of many, sometime hundreds of cell pairs, and an ED system may consist of many stacks. Each membrane stack has a DC (direct current) anode at one end of the stack and a DC cathode at the other end. Under a DC voltage, ions move to the electrode of opposite charge.
A cell pair consists of two types of cells, a diluting cell and a concentrating cell. As an illustrative example, consider a cell pair with a common cation transfer membrane wall and two anion transfer membrane walls forming the two cells. That is, a first anion transfer membrane and the cation transfer membrane form the dilute cell and the cation transfer membrane and a second anion membrane form the concentrating cell. In the diluting cell, cations will pass through the cation transfer membrane facing the anode, but be stopped by the paired anion transfer membrane of the concentrating cell in that direction facing the cathode. Similarly, anions pass through the anion transfer membrane of the diluting cell facing the cathode, but will be stopped by the cation transfer of membrane of the adjacent pair facing the anode. In this manner, salt in a diluting cell will be removed and in the adjacent concentrating cells cations will be entering from one direction and anions from the opposite direction. Flow in the stack is arranged so that the dilute and concentrated flows are kept separate and a desalinated water stream is produced from the dilute flow.
In the ED process, material commonly builds up at the membrane surface in the direction of the electric field, which can, and usually does reduce process efficiency. To combat this effect, Electrodialysis reversal (EDR) was developed and is the primary method of use presently. In EDR, the electrodes are reversed in polarity on a regular basis, for example, every fifteen to sixty minutes. The dilute and concentrate flows are simultaneously switched as well, the concentrate becoming the dilute flow and vice versa. In this way fouling deposits are removed and flushed out.
Once the concentration in the dilution cells falls to lower than about 2000 milligrams/liter (mg/l), electrical resistance is at a level that power demand becomes increasing expensive. To overcome this, and to be able to produce high quality water, electrodeionization (EDI), sometimes called continuous electrodeionization (CEDI) was developed. In this method the cells are filled with ion exchange media, usually ion exchange resin beads. The ion exchange media is orders of magnitude more conductive than the solution. The ions are transported by the beads to the membrane surface for transfer to the concentrate cells. EDI is capable of producing purer water then ED at less power when the feed concentration is reduced sufficiently.
ED processes for water desalination have advantages over RO. They require less pretreatment which will reduce operating costs. They will have higher product water recovery and a higher brine concentration, i.e., there is less brine to dispose.
Univalent selective or monovalent selective membranes primarily transfer monovalent ions. Monovalent selective cation transfer membranes primarily transfer sodium, potassium, etc. Likewise, monovalent selective anion membranes transfer ions such as chloride, bromide, nitrate etc.
Reverse osmosis (RO) dominates the production of fresh water from seawater by membrane processes. While electrodialysis (ED) is used for brackish water and waste water desalination, it is generally considered too expensive for seawater use. In order to be competitive with RO, ED and EDR will require membrane resistance of less than 1 ohm-cm2, preferably less than 0.8 ohm-cm2, and most preferably less than 0.5 ohm-cm2. Ion permselectivity of greater than 90%, more preferably greater than 95%, and most preferably greater than 98% is desired. The membrane has to have long service life, and be physically strong and chemically durable and be low cost.
Reverse electrodialysis (RED) converts the free energy generated by mixing the two aqueous solutions of different salinities into electrical power. The greater the difference in salinity, the greater the potential for power generation. For example, researchers have studied RED using Dead Sea water and fresh or seawater. Researchers in Holland have mixed river water entering the sea and seawater. RED membranes preferably will have a low electrical resistance and a high co-ion selectivity and long service life time, acceptable strength and dimensional stability and, importantly, low-cost.
The polymer electrolyte membrane (PEM) is a type of ion exchange membrane that serves both as the electrolyte and as a separator to prevent direct physical mixing of the hydrogen from the anode and oxygen supplied to the cathode. A PEM contains negatively charged groups, usually sulfonic acid groups, attached or as part of the polymer making up the PEM. Protons migrate through the membrane by jumping from one fixed negative charge to another to permeate the membrane.
PEM's requirements include chemical, thermal and electrochemical stability, and adequate mechanical stability and strength when swollen and under mechanical stress. Other requirements include low resistance, low or preferably no methanol transport in direct methanol fuel cells (DMFC), and low cost.
Development of ion exchange membranes requires an optimization of properties in order to overcome competing effects. Ion exchange membranes for water desalination traditionally have had to meet four main characteristics to be considered successful. These are;                Low electrical resistance to reduce potential drop during operation and to increase energy efficiency        High permselectivity—that is, high permeability to counter-ions but approximately impermeable to co-ions        High chemical stability—ability to withstand pH from 0 to 14 and oxidizing chemicals        Mechanical strength—The membrane must be able to withstand the stresses of being handled while being manufactured into a module or other processing device. The membrane must also have good dimensional stability in operation and not swell or shrink excessively when the fluid contacting it changes concentration or temperature.        
Membrane developers have recognized that for a given chemistry used to make an ion exchange membrane, a thinner membrane would give a lower resistance and also allow more membrane area per unit volume of device. However, thinner membranes are more susceptible to dimensional change from environmental effects, such as changes in ionic concentration of the fluids contacting them or operating temperature changes. Moreover, to develop and produce defect-free membranes is more difficult for the case of thinner membranes because there is less margin of error during membrane production as there is for thicker membranes where the membrane thickness covers over defects that may occur in membrane formation.
U.S. Pat. No. 7,226,646 describes an ion conducting membrane comprising an ion conducting region and a non-ion conducting region. The ion conducting region is formed by a plurality of passageways extending through the membrane filled with ion conducting material. The passageways may be formed in the substrate sheet by a number of methods, such as drilling, chemical etching, or punching, to provide straight-through passageways. Preferred arrangements of passageways are square, rectangular, triangular or hexangular arrays. Ionogenic polymers are deposited in the passageways to make up the ion conducting regions. In some embodiments a skin is bonded to the surface of the substrate, or coated onto one or both surfaces.
U.S. Pat. No. 7,649,025 describes a composite ion exchange membrane comprising a support membrane and ion exchange resin composition within the pores of the substrate. The ion exchange resin is a specific class of aromatic polyethers. In related U.S. Pat. No. 7,537,852, the porous membrane is a polybenzazole membrane.
U.S. Pat. No. 7,550,216 describes a composite solid polymer electrolyte membrane comprising a porous polymer substrate interpenetrated with a water soluble ion-conducting material. The porous polymer substrate comprises a homopolymer or copolymer of a liquid crystalline polymer such as such as a polybenzazole (PBZ) or polyaramid polymer. Preferred polybenzazole polymers include polybenzoxazole (PBO), polybenzothiazole (PBT) and polybenzimidazole (PBI) polymers. Preferred polyaramid polymers include polypara-phenylene terephthalamide (PPTA) polymers. In other preferred embodiments, the polymer substrate comprises a thermoplastic or thermoset aromatic polymer. The ion-conducting material comprises a water soluble homopolymer or water soluble copolymer of at least one of a sulfonated ion-conducting aromatic polymer.
W. L. Gore & Associates, Inc. (Newark, Del.) describes, in U.S. Pat. No. 6,254,978, a integral composite membrane having a porous polymeric membrane impregnated with a perfluoro ion exchange material to make the micropores of the membrane occlusive and a surfactant having a molecular weight greater than 100 wherein the thickness of the composite membrane is less than 0.025 mm. U.S. Pat. No. 5,547,551 describes a composite membrane comprising expanded polytetrafluoroethylene membrane support impregnated with ion exchange material, having a total thickness of less than 0.025 mm. U.S. Pat. Nos. 5,599,614 and 5,635,041 also describe composite membranes comprising microporous expanded PTFE substrates impregnated with Nafion® (E.I. DuPont Wilmington Del.). Gore-Select® membranes (W.L. Gore & Associates, Inc., Elkton, Md.) are composite membranes comprising a microporous expanded PTFE membrane having an ion exchange material impregnated therein.
U.S. Pat. No. 6,110,333 describes a composite membrane comprising an ion exchange polymer and a support of expanded polytetrafluoroethylene polymer having a porous microstructure of polymeric fibrils, said expanded polytetrafluoroethylene polymer being at least about 85% crystalline.
U.S. Pat. No. 6,689,501 describes a composite membrane for use in a fuel cell membrane electrode assembly comprising a porous polymeric substrate and a cation exchange material impregnant partially filling the substrate such that the substrate comprises a first region having pores substantially filled with the impregnant, and a second substantially porous region. The cation exchange material covers one surface of the substrate in a dense surface layer, contiguous with the first region of the substrate. The substrate has greater than 10% residual porosity, and the composite membrane is substantially gas impermeable and has at least one substantially porous major surface. U.S. Pat. No. 5,985,942 describes composite membranes comprising a porous substrate and ion exchange materials comprising substituted trifluorostyrene polymers and copolymers.
McMaster University has two US patents related to composite membranes having polyelectrolytes or hydrogels bonded or crosslinked around porous support structural elements. U.S. Pat. No. 6,258,276 discloses charged membranes comprising a porous substrate and a cross-linked polyelectrolyte or hydrogel located in the pores of the substrate. The patent discloses polymerization in the substrate pores of a monomer or a mixture of monomers with a cross-linking agent, the monomer or at least one of the monomer mixture being selected from those monomers which contain a functional group that provides an ion-exchange site and those which contain a group which is susceptible to a chemical reaction by which such functional groups are subsequently introduced to the in situ-formed polymer. Alternatively, an uncrosslinked polyelectrolyte or hydrogel may be formed in the pores of the substrate as described and subsequently crosslinked.
U.S. Pat. No. 7,247,370 provides for an asymmetric membranes composed of a microporous substrate whose pores contain a crosslinked gel, preferably a hydrogel or a polyelectrolyte gel, located in pores of the substrate, where the density of the crosslinked gel is greater at or adjacent to one major surface of the membrane than the density at the other major surface so that there is a gradient of gel distribution from one major surface of the membrane towards the other major surface of the membrane.
U.S. Pat. No. 5,510,394 discloses a process where a solid polymeric sheet which has been cast or extruded with a fixed percentage of a high boiling point ester plasticizer is then immersed or otherwise contacted with one or more monomers along with a small fraction of a crosslinking bifunctional monomer such as divinyl benzene. The monomers exchange with the high boiling point plasticizers and are polymerized within the interstices of the base films. The monomers may be ion containing monomers, or monomers which can be converted after polymerization into an ion exchange membranes by for example, sulfonation of phenyl groups or amination by tertiary amines of chloromethyl groups attached to aromatic polymers.
WO2010/013861 describes an anion exchange composite membrane that is produced by impregnating a porous film with a styrene-based monomer, a vinylbenzene-based monomer, a crosslinking agent and an initiator, and after polymerization, functionalizing the resulting crosslinked polymer by adding ammonium ions.
Membranes having charged polymers are known. Charged membranes, usually negatively charged, are used for nanofiltration. Such membranes are made to have a high water permeability. Such membranes would not be suitable for ED as they would have high osmotic flow due to their high water permeability. This effect would give poor co-ion rejection. Membranes for fuel cells are designed to transport hydrogen ions and are not suitable for transferring larger ions.